If one takes into account the many constraints (cost, post office approval, customer requirements, mechanical requirements, human readability) that must be simultaneously satisified, it may fairly be said that it is not easy to print postage. For nearly a hundred years, companies such as Hasler (a predecessor of the assignee of the present invention) and its competitors have provided postage meters which print postage by means of mechanical relief die plates. Generations of mechanical engineers have developed and refined the art of mechanical printing of postage so that today's postage meters (also called franking machines) offer a high-quality die-printed postage indicium together with all the benefits flowing from the use of microprocessors.
It has been recently suggested to use digitally formed indicia instead of die-printed indicia, a move which would discard a substantial fraction of the accumulated experience with die printing of postage and which opens up a host of new problems. The printing technologies most often proposed for digitally formed indicia are ink-jet and laser printing. These technologies have many potential disadvantages. A chief disadvantage is that while it is easy to print on plain paper, on empty envelopes or on conventional labels, it is difficult to print on mail pieces of varying thickness. A further problem is that mail pieces may be constructed from a variety of materials including smooth paper, rough paper, and nonwoven fabrics such as Tyvek, and these materials differ greatly from one to the next in their suitability for laser or ink-jet printing. Another problem comes from the inks commonly used for ink-jet printing. Many such inks are water-soluble and thus are easily smeared or damaged as a mail piece passes through the mail stream.
The hazards faced by a postal indicium that is in the nature of a bar code are of a new and different sort as compared with the hazards faced by a die-printed indicium. With a bar code, especially a two-dimensional bar code as has recently been suggested, the loss of even a small portion of the code can make it difficult or impossible to read the entirety of the code. The problems of printing on varying materials of varying thicknesses exacerbate the risk of loss of a portion of the bar code. Abrasion during shipment, or water damage, can also damage the code. Finally, some materials such as nonabsorbent substrates don't take ink-jet ink well.
Yet another problem with digitally formed postal indicia is that if the indicium turns out to be defective and if the defect is not detected until the mail piece has entered the mail stream, then there will be unwanted consequences such as returning the mail piece to sender or forwarding the mail piece with postage due. One category of risk is that a digital printer such as an ink-jet printer could run low on ink, or develop a clogged jet, leading to an indicium which is not visibly flawed but which might nonetheless fail the cancellation checking by the postal service. Another category is that an indicium might pass a test at the sender's location and yet fail the test at the postal service due to a mechanical assult or water damage, so that a test at the sender's location would not provide complete confidence that the mail peice will not be returned to sender or delivered with postage-due.
There is thus a great need for an apparatus that prints digitally formed postal indicia on mail pieces, that is robust against abrasion and physical assaults, that is resistant to water damage, and that works well with a wide variety of materials and thicknesses. There is also a great need for an apparatus which permits a test of the indicium at the customer location, and which permits a high confidence that a favorable test at the customer location presages a favorable test at the postal service location. Finally there is a need for a means of printing digitally formed indicia on mail pieces that saves "spoiled" mail pieces in the event of a defective indicium.